In food, pharmaceutical and other industries, sanitary stainless steel pipes are used as process pipes for conveying raw materials and finished products, and the welding quality has always been the top priority in the quality control process. Defects such as undercut, incomplete penetration, etc., these defects will cause the medium to accumulate residues in the pits on the inner wall of the pipeline during transportation, and the medium remaining on the inner wall of the pipeline will rot and deteriorate over time, which is very hygienic. The food and pharmaceutical industries are absolutely not allowed, so the requirements for the internal and external surface forming of this sanitary stainless steel pipeline weld are very high, requiring single-sided welding and double-sided forming, and defects such as undercut and incomplete penetration are not allowed. There are strict requirements for the height of the weld seam and the depth of the pit on the inner wall.
Installation requirements for piping in design documents
The installation requirements of the design documents for the pipeline: the stainless steel pipeline should be welded by argon arc welding, the height of the weld on the outer surface of the pipeline should be controlled at 0mm-0.8mm, the depth of the weld pit on the inner wall of the pipeline should be controlled at 0mm-0.2mm, and the height of the inner wall should be controlled at 0mm-0.2mm. At 0-0.3mm, all stainless steel pipes should be cut with a flat end machine, and the burrs should be removed after cutting. The outer surface of the stainless steel pipe should be mechanically polished (polished with a wool wheel and polishing paste after sanding). The inner wall of the pipe is in After the installation is completed, use a certain proportion of acid for circulating flushing until the design requirements are met.
Welding technical requirements
Welding of stainless steel pipes shall be carried out in accordance with the provisions of the JB4708 standard. When welding stainless steel materials, the welding characteristics of stainless steel pipes must be considered, and the ferrite content of the weld must be strictly controlled to ensure the welding quality, and the welding must be carried out according to the welding process requirements.
In the welding process, if you encounter a dead angle of installation, you must use quick-connect joints; try to minimize the use of quick-connect joints in places that are convenient for installation, which can reduce the number of welding points.
In order to ensure the quality of the welding seam, after each welding, it should be carefully polished, the width of the polished groove should be uniform, and the unevenness should be polished flat. This is not easy to produce defects, and the forming is also good (this is suitable for wire welding, and natural fusion welding is not required).
Since the design documents have high requirements for pipeline welding, it is necessary to compare the welding process before welding the pipeline. Below I will briefly introduce several key welding parameters in the welding process.
1. Selection of welding method
The selection of the welding method should be determined according to the material and quality requirements of the workpiece to be welded. Since they are all thin-walled sanitary stainless steel pipes with small diameter, the general diameter is DN50 and below, and the welding seam requires single-sided welding and double-sided forming. , and considering that the on-site process pipelines are arranged in rows, many welding positions cannot be completed by automatic welding machines, so according to the actual situation, the overall argon-filled manual tungsten argon arc welding welding process in the tube is selected. Since argon is an inert gas, argon is used to isolate the air during the welding process, so that the metal to be welded is not easily oxidized, and the welding seam can be beautifully formed, smooth and burr-free.
2. Shielding gas flow, welding current and voltage
Theoretically, the larger the gas flow rate, the stronger the protective layer's ability to resist the influence of air. However, in practice, it has been found that when the gas flow rate is too large, the protective layer will produce irregular flow, which will cause the air to be involved and reduce the The protection effect affects the quality of the weld and also wastes gas, so the flow rate of argon must be selected according to the actual situation during welding.
3. The tube is filled with argon for protection
The purity of the argon gas used needs to be at least 99.99% or more. When the tube is filled with argon gas for protection, 15 seconds to 20 seconds before the arc strike, start the switch air valve to remove the air in the tube, and adjust the flow rate required by the flow meter. Small pipes can be filled with argon, and large-diameter pipes can be filled with argon locally to reduce the amount of argon. After the welding is completed, the delivery of argon should be stopped for at least 10 seconds.
Post-weld treatment
In order to improve and ensure the corrosion resistance of the stainless steel weld and the surface of the heat affected zone, the stainless steel pipe must be subjected to alkaline cleaning and passivation treatment after welding (not required for the gas pipeline).
Since the welding of sanitary stainless steel pipes is more demanding than other pipe welding, it needs to pass GMP certification before production. After the traditional Chinese medicine extraction project of Tonghua Dongbao Pharmaceutical Co., Ltd., manual argon arc welding is used, and the qualified rate of welding can reach 99.8%. , Compared with automatic welding, manual argon arc welding can be fully qualified for the welding of sanitary stainless steel pipes. The welding process of manual tungsten argon arc welding with argon in the pipe can also obtain high-quality welded joints. The welding seam is beautiful in shape and smooth inside and outside. No burr, can meet GMP certification requirements and production process